1. Field of the Invention
The present invention generally relates to a temperature-sensing circuit using CMOS (complementary metal oxide semiconductor) transistors, and particularly relates to a temperature-sensing circuit capable of performing a stable operation at a high temperature, notably at a temperature above 100° C.
2. Description of the Related Art
There are various types of temperature sensors known in the art.
A first type is a temperature sensor system making use of thermocouples or resistors. In the case of thermocouples, a thermocurrent produced at a contact point between two types of metal wires is amplified by an amplifier and measured by a voltmeter. In the case of resistors, a three-wire bridge arrangement is used such that a voltage change due to the change of resistance is amplified by an amplifier and a change of resistance value due to a change of temperature is measured by a voltmeter.
A second type is a semiconductor temperature sensors using bipolar transistors (see “Transistor technology”, CQ publishing, October 1990, p. 469). Such sensors rely on the characteristic that the base-emitter voltage changes linearly in response to the change of temperature. Therefore, the semiconductor temperature sensors are widely used since they reduce production variation and give good accuracy and reproducibility.
A third type is a semiconductor temperature sensor using MOS transistors. For example, it is described in JP-A 9-243466 that a voltage gain β of the MOS transistor is converted into a voltage value and the converted voltage is output as a value representing temperature. JP-A 7-321288 discloses a temperature sensor based on a bipolar transistor similar to a technique described above, but using NPN transistors to achieve a CMOS transistor.
The above-mentioned first type temperature sensor system either using thermocouples or resistors generates a small output. Therefore, in order to amplify the output, it is necessary to connect a high-performance amplifier to the temperature sensor system. Such a complicated electronic circuit results in increased cost and size of the temperature sensor system.
The above-mentioned second type temperature sensor is constructed using a bipolar transistor and thus cannot be integrated into an IC chip fabricated by a standard CMOS process. Also, since a bipolar transistor is a current element controlled by an electric current, it is difficult to construct a circuit with a low current consumption.
Among the temperature sensors of the above-mentioned third type, the temperature sensor disclosed in JP-A 9-243466 is susceptible to process fluctuation since it uses the voltage gain value β. The temperature sensor disclosed in JP-A 7-321288 has a drawback in that it is difficult to form a circuit of low current consumption since the bipolar transistor similar to that used in the second type temperature sensor is a current element controlled by an electric current.
For all of the first, second and third types of temperature sensors, the upper limit of a range within which an accurate sensing can be guaranteed is about 100° C., at which point a reverse leakage current increases at a pn junction. Accuracy of temperature measurement rapidly drops at a temperature higher than the upper limit.